But they are in line with previous studies, which have linked head trauma and certain toxins - along with family history and other environmental exposures - to the disease.

"I think all of us are beginning to realize that there's not one smoking gun that causes Parkinson's disease," said Dr. James Bower, a neurologist from the Mayo Clinic in Rochester, Minnesota who wasn't involved in the new research.

"There might be many paths to the ultimate development of Parkinson's disease," he told Reuters Health.

For example, Bower said, some people who are genetically predisposed might need just one "environmental insult" - such as a blow to the head - to set them up for Parkinson's. Others who aren't naturally susceptible to the disorder could still develop it after multiple exposures.

Head trauma and contact with pesticides "may not be directly related, and may be two independent stresses," Columbia University neurologist David Sulzer, who also wasn't part of the study team, told Reuters Health in an email.

About 50,000 to 60,000 older adults in the U.S. are diagnosed with Parkinson's disease each year, according to the National Parkinson Foundation.

For the new study, researchers led by Pei-Chen Lee from the University of California at Los Angeles compared 357 people with a recent Parkinson's diagnosis to a representative sample of 754 people without the disease, all living in central California, which is a major agricultural region.

The study team asked all of them to report any past traumatic head injuries - in which people had been unconscious for at least five minutes - and used their home and work addresses to determine their proximity to pesticide sprayings since 1974.

Those surveys showed that close to 12 percent of people with Parkinson's had been knocked unconscious, and 47 percent had been exposed to an herbicide called paraquat near both their home and workplace.

That's in comparison to almost seven percent of control-group participants with a history of head injury and 39 percent with pesticide exposure.

On their own, traumatic brain injury as well as living and working near pesticide sprayings were each tied to a moderately increased risk of Parkinson's disease. Combined, they were linked to a tripling of that risk, the researchers reported Monday in the journal Neurology. That was after taking into account people's baseline risk based on their age, gender, race, education, smoking history and family history of Parkinson's.

Lee's team didn't know which came first in people who'd had both head trauma and paraquat exposure.

It makes sense, the researchers noted, that a head injury would increase inflammation in the brain and disrupt the barrier that separates circulating blood and brain fluid. Those changes could then make neurons in the brain more vulnerable to the effects of pesticides, ultimately increasing the risk of Parkinson's.

But that's just a theory.

"There are all kinds of hypotheses," Bower said. But the study "is more evidence that traumatic injury to the brain can lead to later problems that are usually neurodegenerative," he added. "We need to be increasingly careful about preventing these traumatic brain injuries."